Yeah, but Twilight has been wrong about magic before. Almost consistently, in fact, for somepony who allegedly studies it. To be honest I take a lot of what's
said in the show with a bit of a grain of salt. If it's show, that one thing, but if it's only said... well, they could be lying, or wrong.
It just doesn't make any sense that a unicorn can only have certain kinds of magic, and then all of a sudden a pony comes along and say "oop, looks like I can do
every kind of magic!" It feels almost mary sue-ish. Especially when you consider how often Twilight lectures on how
Magic A is Magic A, for her to come along and suddenly be magic B just doesn't make sense in universe. I think it's still possible that when she said that she meant that they start off with a bit of magic related to their particular knack. Especially since I can't think of
any way the system could work that some ponies would just get a spell, but then other ponies would have to work for their spells, but could get any one. It only makes sense if
any pony can get
any spell, but have a hard time of it. Like Jteeth said - some people may have a knack for certain feilds, but any person can pick up any skill if they try hard enough.